by Keyword: Porosity

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Diez-Escudero, A., Espanol, M., Beats, S., Ginebra, M. P., (2017). In vitro degradation of calcium phosphates: Effect of multiscale porosity, textural properties and composition Acta Biomaterialia 60, 81-92

The capacity of calcium phosphates to be replaced by bone is tightly linked to their resorbability. However, the relative importance of some textural parameters on their degradation behavior is still unclear. The present study aims to quantify the effect of composition, specific surface area (SSA), and porosity at various length scales (nano-, micro- and macroporosity) on the in vitro degradation of different calcium phosphates. Degradation studies were performed in an acidic medium to mimic the osteoclastic environment. Small degradations were found in samples with interconnected nano- and micropores with sizes below 3 µm although they were highly porous (35–65%), with maximum weight loss of 8 wt%. Biomimetic calcium deficient hydroxyapatite, with high SSA and low crystallinity, presented the highest degradation rates exceeding even the more soluble β-TCP. A dependence of degradation on SSA was indisputable when porosity and pore sizes were increased. The introduction of additional macroporosity with pore interconnections above 20 µm significantly impacted degradation, more markedly in the substrates with high SSA (>15 m2/g), whereas in sintered substrates with low SSA (<1 m2/g) it resulted just in a linear increase of degradation. Up to 30 % of degradation was registered in biomimetic substrates, compared to 15 % in β-TCP or 8 % in sintered hydroxyapatite. The incorporation of carbonate in calcium deficient hydroxyapatite did not increase its degradation rate. Overall, the study highlights the importance of textural properties, which can modulate or even outweigh the effect of other features such as the solubility of the compounds. Statement of Significance The physicochemical features of calcium phosphates are crucial to tune biological events like resorption during bone remodeling. Understanding in vitro resorption can help to predict the in vivo behavior. Besides chemical composition, other parameters such as porosity and specific surface area have a strong influence on resorption. The complexity of isolating the contribution of each parameter lies in the close interrelation between them. In this work, a multiscale study was proposed to discern the extent to which each parameter influences degradation in a variety of calcium phosphates, using an acidic medium to resemble the osteoclastic environment. The results emphasize the importance of textural properties, which can modulate or even outweigh the effect of the intrinsic solubility of the compounds.

Keywords: Calcium phosphates, Degradation, Porosity, Textural properties

Malandrino, Andrea, Lacroix, Damien, Hellmich, Christian, Ito, Keita, Ferguson, Stephen J., Noailly, J., (2014). The role of endplate poromechanical properties on the nutrient availability in the intervertebral disc Osteoarthritis and Cartilage , 22, (7), 1053-1060

Objective To investigate the relevance of the human vertebral endplate poromechanics on the fluid and metabolic transport from and to the intervertebral disc (IVD) based on educated estimations of the poromechanical parameter values of the bony endplate (BEP). Methods 50 micro-models of different BEP samples were generated from μCTs of lumbar vertebrae and allowed direct determination of porosity values. Permeability values were calculated by using the micro-models, through the simulation of permeation via computational fluid dynamics. These educated ranges of porosity and permeability values were used as inputs for mechano-transport simulations to assess their effect on both the distributions of metabolites within an IVD model and the poromechanical calculations within the cartilaginous part of the endplate i.e., the cartilage endplate (CEP). Results BEP effective permeability was highly correlated to local variations of porosity (R2 ≈ 0.88). Universal patterns between bone volume fraction and permeability arose from these results and from other experimental data in the literature. These variations in BEP permeability and porosity had negligible effects on the distributions of metabolites within the disc. In the CEP, the variability of the poromechanical properties of the BEP did not affect the predicted consolidation but induced higher fluid velocities. Conclusions The present paper provides the first sets of thoroughly identified BEP parameter values that can be further used in patient-specific poromechanical studies. Representing BEP structural changes through variations in poromechanical properties did not affect the diffusion of metabolites. However, attention might be paid to alterations in fluid velocities and cell mechano-sensing within the CEP.

Keywords: Bony endplate, Spine mechanobiology, Intervertebral disc metabolites, Hydraulic Permeability, Bone Porosity, Poromechanics

Montufar, Edgar B., Traykova, Tania, Planell, Josep A., Ginebra, Maria-Pau, (2011). Comparison of a low molecular weight and a macromolecular surfactant as foaming agents for injectable self setting hydroxyapatite foams: Polysorbate 80 versus gelatine Materials Science and Engineering: C 31, (7), 1498-1504

Hydroxyapatite foams are potential synthetic bone grafting materials or scaffolds for bone tissue engineering. A novel method to obtain injectable hydroxyapatite foams consists in foaming the liquid phase of a calcium phosphate cement. In this process, the cement powder is incorporated into a liquid foam, which acts as a template for macroporosity. After setting, the cement hardens maintaining the macroporous structure of the foam. In this study a low molecular weight surfactant, Polysorbate 80, and a protein, gelatine, were compared as foaming agents of a calcium phosphate cement. The foamability of Polysorbate 80 was greater than that of gelatine, resulting in higher macroporosity in the set hydroxyapatite foam and higher macropore interconnectivity. Gelatine produced less interconnected foams, especially at high concentrations, due to a higher liquid foam stability. However it increased the injectability and cohesion of the foamed paste, and enhanced osteoblastic-like cell adhesion, all of them important properties for bone grafting materials.

Keywords: Hydroxyapatite, Porosity, Calcium phosphate cement, Scaffolds, Foaming, Bone regeneration

Charles-Harris, M., Koch, M. A., Navarro, M., Lacroix, D., Engel, E., Planell, J. A., (2008). A PLA/calcium phosphate degradable composite material for bone tissue engineering: an in vitro study Journal of Materials Science-Materials in Medicine , 19, (4), 1503-1513

Biodegradable polymers reinforced with an inorganic phase such as calcium phosphate glasses may be a promising approach to fulfil the challenging requirements presented by 3D porous scaffolds for tissue engineering. Scaffolds' success depends mainly on their biological behaviour. This work is aimed to the in vitro study of polylactic acid (PLA)/CaP glass 3D porous constructs for bone regeneration. The scaffolds were elaborated using two different techniques, namely solvent-casting and phase-separation. The effect of scaffolds' micro and macrostructure on the biological response of these scaffolds was assayed. Cell proliferation, differentiation and morphology within the scaffolds were studied. Furthermore, polymer/glass scaffolds were seeded under dynamic conditions in a custom-made perfusion bioreactor. Results indicate that the final architecture of the solvent-cast or phase separated scaffolds have a significant effect on cells' behaviour. Solvent-cast scaffolds seem to be the best candidates for bone tissue engineering. Besides, dynamic seeding yielded a higher seeding efficiency in comparison with the static method.

Keywords: Biocompatible Materials/ chemistry, Bone and Bones/ metabolism, Calcium Phosphates/ chemistry, Cell Differentiation, Cell Proliferation, Humans, Lactic Acid/ chemistry, Microscopy, Confocal, Microscopy, Electron, Scanning, Osteoblasts/metabolism, Permeability, Polymers/ chemistry, Porosity, Solvents/chemistry, Tissue Engineering/ methods

Koch, M. A., Engel, E., Planell, J. A., Lacroix, D., (2008). Cell seeding and characterisation of PLA/glass composite scaffolds for bone tissue engineering Journal of Biomechanics 16th Congress, European Society of Biomechanics , Elsevier (Lucerne, Switzerland) 41, (Supplement 1), S162

In this study polymer-glass composite scaffolds were characterized by permeability and porosity, two important properties for the use in perfusion bioreactors. These scaffolds were seeded with osteoblast-like cells to assess the efficiency of the used bioreactor. The used PLA/glass composite scaffolds are adequate for the perfusion culture. The high porosity and pore interconnectivity allow an even cell distribution and incorporation of a high cell number. For optimisation of the perfusion bioreactor system, further research has to be dedicated to the cell seeding and culture.

Keywords: Biomedical materials, Bioreactors, Bone, Cellular biophysics, Composite materials, Orthopaedics, Permeability, Polymers, Porosity, Porous materials, Tissue engineering